Air turbine starters are well known in the aviation field and are commonly used to start gas turbine engines. A common clutch type often used in air turbine starters is the pawl and ratchet type, (see FIG. 3). This type of clutch includes a ratchet 6 which is driven by the turbine of the starter through gearbox reduction gearing. A pawl carrier 8 surrounds the ratchet 6 and has a plurality of pawls 2 mounted thereto. Each of the pawls 2 is spring-loaded into engagement with the ratchet 6 by a leaf spring 4. The torque generated by the starter's turbine is transmitted through the ratchet 6 to the pawls 2 and then to the pawl carrier 8. The pawl carrier 8 transmits this torque starter to the starter's output shaft which drives the gas turbine engine.
During a normal start sequence of a gas turbine engine, the starter and engine are initially at rest. As the turbine of the starter begins to rotate, the ratchet 6 begins to rotate, engaging the head portion of each of the pawls 2. At this point, the starter is driving the engine. As the starter and engine accelerate, centrifugal force begins to counteract the force of the leaf springs 4 which are in sliding, frictional engagement with the tails of the pawls 2. After starter cutout, the engine accelerates on its own. The pawl carrier 8, which is now driven by the engine through the output shaft of the starter, accelerates allowing the pawls 2 to completely disengage from the ratchet 6 due to the centrifugal force. The starter's turbine and the ratchet 6 then coast to rest, while the engine accelerates to engine idle speed. Centrifugal force on the pawls 2 prevent the pawls 2 from re-engaging the ratchet 6 until the engine is shut down and the engine speed is low enough for the spring force to overcome the centrifugal force.
In the case of an inflight or ground restart attempt, the starter is initially at rest and the engine is decelerating. A restart may be made if the engine speed is below the pawl return speed. The pawl return speed is the rotational speed of the engine at which the spring force overcomes the centrifugal force and the springs 4 begin to force the pawls 2 into engagement with the ratchet 6. At the pawl return speed the pawls 2 will be ratcheting over the ratchet 6 because the starter is at rest. Once started, the turbine of the starter and the ratchet 6 accelerate rapidly until the ratchet rotational speed equals the pawl rotational speed. Engagement between the pawls 2 and the ratchet 6 occurs and the starter begins to accelerate the engine. The remaining operation is identical to the normal start sequence.
Because modern aircraft engines are larger than their predecessors, they require greater assistance from the air turbine starter. To meet this requirement, the starter must drive the engine almost to its idle speed. Consequently, pawl return speeds have risen and the pawl and ratchet clutches used to start these modern engines must be able to transmit larger loads then previously required. To transmit these loads the size and mass of the pawls has been increased which in turn has required greater spring forces to force the pawls into engagement. As a result, the leaf springs in the clutch are exposed to greater stresses increasing the frictional engagement between the leaf spring and the pawls, and between the individual spring leaves. Not surprisingly, on starters used with these modern engines an increase in the spring wearout rate has been observed.
Another disadvantage associated with leaf springs, as used in the prior art clutch, is that each pawl requires a separate leaf spring. Because the pawls rotate independent from each other, during a running engagement, some of pawls may engage the ratchet earlier than other pawls resulting in a single or double pawl engagement and generating an eccentric load within the clutch.
Also, because the forces on the springs and pawls must be balanced to provide the necessary disengage/reengage speed, the manufacturing tolerances on leaf springs must be held very close, making their manufacturing expensive. For the newer and larger pawls it is very difficult to manufacture the leaf springs to provide the required spring forces.
Accordingly, a need exists for a pawl and ratchet clutch and an air turbine starter having such a clutch in which the disadvantages associated with leaf springs are eliminated.